1. Field of the Invention
The present invention relates to a microcomputer, and more particularly, to an instruction decoder provided therein.
2. Description of the Prior Art
As shown in FIG. 1, an instruction decoder 2 decodes a data provided from a read only memory (ROM) 1 for a program and outputs an instruction. In forming an instruction, when the instruction data is, for example, an 8-bit data, the number of instructions in an instruction map which can be made with 8-bit data is automatically determined, for example, to be 100.
A specific device which is controlled by the output of the instruction decoder is a multiplexer provided in a microcomputer. There are many combinations of operations of multiplexers, and accordingly, the number of devices provided in a microcomputer which can be operated by instructions is much larger than the number of instructions which are outputs of the instruction decoder. The instruction decoder is different in function from the ROM and a data table ROM; however, it is the same as that of a ROM in structure.
Since the instruction decoder provided in a conventional microcomputer is constituted only by a ROM as described above, the number of instructions which can be formed by one instruction decoder is insufficient. In order to form a substantially large number of instructions, it is necessary to execute the instruction forming operation process in stages (specifically, a plurality of stages of decoding operations are necessary with respect to one instruction data input). A method in which a large number of instructions are formed by an instruction decoder of such a plural stage structure is generally called a complex instruction set computer (CISC) method. The CISC method has drawbacks because the instruction speed is low.
A reduced instruction set computer (RISC) method is a method which is developed from a viewpoint of increasing the instruction speed. The RISC method, in which instructions are extremely simplified, has drawbacks because instructions are so simplified that without a compiler, it is difficult to make the most of the capability of the microcomputer.